1. Field of the Invention
This invention relates to a control apparatus for a DC motor, or more in particular to a control apparatus suitable for applications such as an elevator requiring an accurate speed control.
Most DC motors for driving elevator cars are controlled by the Ward-Leonard system. The static Leonard system employing a thyristor converter has long been used as a practical means for control of the DC motor for the rolling mill. In recent years, this static Leonard system has found applications or has been tested for practical applications in the field of the elevator.
There are two types of the static Leonard system. One is a system in which a bilateral thyristor converter is connected to the armature while a unidirectional thyristor converter is connected to the field system. The other static Leonard system is such that a unidirectional thyristor converter is connected to the armature whereas a bilateral thyristor converter is connected to the field system.
This invention relates to an improvement in the motor control apparatus of the latter type.
2. Description of the Prior Art
Not only such a load as an elevator car but also other loads, which are driven by an electric motor, require a large torque for acceleration or deceleration. Since the acceleration or deceleration period is short, however, it is possible to cause the motor to operate above its rating for such a period. In fact, the motor for the elevator car is so controlled that 200 to 300% of armature current flows as required during the acceleration or deceleration.
In the case where a unidirectional thyristor converter is connected to the armature to permit unidirectional current flow while connecting a bilateral thyristor converter to the field system to allow current flow in both directions for the control of a DC motor, it is well known to control both the armature current and the field current at the same time in accordance with a torque command representing a speed error. To control simultaneously the armature current and the field current which are both probable causes of an external disturbance, however, is undesirable in view of the resulting non-linearity of the torque generation characteristics of the DC motor with respect to the torque command.
On the other hand, a method obviating this disadvantage is also well known in which the armature current is controlled at a fixed level which is different for acceleration/decleration and for the other periods, so that the torque generation is controlled by varying the field current, as is disclosed in detail in U.S. Pat. No. 3,811,079 entitled "DC Motor Control System", patented May, 14, 1974 based on application, Ser. No. 354,459, filed Apr. 25, 1973 in the name of Korefumi Tashiro et al and assigned to the same assignee as the present invention. In this control apparatus, a large torque is not always required for acceleration or deceleration. In spite of this, an armature current as large as twice or thrice the rating is made to flow over the entire period of acceleration or deceleration, thus posing a problem of heat being generated in the motor. Also, during the transient period from steady run to deceleration, the difference between the rise time of the armature current and the time of change in the field current results in the disadvantage that the motor is undesirably accelerated temporarily. Further, in a stage during the transfer from accelerated or decelerated state to the steady run, the armature current and the field current unavoidably undergo a change, thus making it impossible to completely eliminate the non-linearity of the torque characteristics mentioned above.